This invention relates to an injector driving circuit.
With the tendency toward the electronic control of automobiles of these days, electronic fuel injection (EFI) systems have come to be used for direct injection of fuel into combustion chambers of engines instead of using carburetors. FIG. 1A shows a positional relationship between an EFI device 1 and an engine body. FIG. 1B shows an outline of one such EFI system. As shown in FIG. 1B, fuel under pressure is fed from a fuel tank (not shown) through a fuel feed pipe 2, and injected into a combustion chamber 4 of an engine through a valve 6. In conjunction with a solenoid coil 8 controlled by an injector driving circuit 10, the valve 6 constitutes a solenoid valve or solenoid-type injector 1 shown in FIG. 1A for injecting fuel into the combustion chamber 4.
In the EFI system of this type, for example, the period for which the solenoid coil 8 is energized need be set in direct proportion to the amount of fuel injected into the combustion chamber 4. To attain this, it is necessary that the rise of current pulse for enegizing the solenoid coil 8 be made steep so that the solenoid coil 8 may be quickly brought to a pull-on state to promptly displace the valve 6 from its initial position to the operating position as illustrated, and that the fall of the current pulse be made sharp so that the solenoid coil 8 may be quickly brought to a pull-off state to promptly return the valve 6 to the initial position, thereby suddenly stopping the fuel injection. Fuel injection will be delayed if the rise of the energizing current pulse is dull, while fuel cutoff will be delayed if the trailing edge of the current pulse is dull. If the fuel injection and cutoff are thus delayed, then hydrocarbon, carbon monoxide, nitric oxides, etc. contained in exhaust gas will be increased in concentration, or fuel consumption will be increased. Accordingly, the injector driving circuit 10 need be constructed in compliance with these requirements.
FIG. 2 is a circuit diagram of a prior art injector driving circuit. This injector driving circuit includes an npn transistor 12 whose collector is coupled to one end of the solenoid coil 8 the other end of which is coupled to a power supply terminal V.sub.D, whose base is coupled to an input terminal V.sub.IN, and whose emitter is grounded, and a Zener diode 14 coupled in a reverse direction between the one end of the coil 8 and the ground.
When a driving voltage as shown in FIG. 3A is applied to the input terminal V.sub.IN of the driving circuit shown in FIG. 2, the transistor 12 is turned on to cause a current as shown in FIG. 3B to flow through the coil 8. As a result, the coil 8 is brought to the pull-on state to hold the valve 6 in the operating position shown in FIG. 1, for example. Thereafter, when the level of the driving voltage goes low to turn off the transistor 12, the current energy at the coil 8 passes through the Zener diode 14. At the point of time when the driving voltage is reduced to the low level, the collector voltage of the transistor 12 suddenly rises from the ground potential level to the level of a Zener breakdown voltage V.sub.Z, as shown in FIG. 3C, and then drops to the level of a power supply voltage V.sub.D. When the coil current is reduced below a predetermined value, the coil 8 is brought to the pull-off state, and the valve 6 is returned to the initial position by the agency of e.g. a spring (not shown).
In the above-mentioned driving circuit, the time required for the current flowing through the coil 8 when the driving voltage shown in FIG. 3A is applied to reach a predetermined value, that is, a value great enough to displace the valve 6, depends upon the time constant of a circuit including the coil 8 and the transistor 12, and the magnitude of a voltage applied to the power supply terminal V.sub.D. In the circuit shown in FIG. 2, it is impossible to apply a very high voltage to the power supply terminal V.sub.D, so that it takes relatively long time for the coil current to reach the predetermined value, eventually causing the operation of the valve 6 to be delayed. In this case, in order to quickly attain a pull-on state, a current much greater than the necessary current to keep the coil 8 in the pull-on state would flow through the coil 8 and the transistor 12 while the transistor 12 is conducting. If a resistor is coupled to the coil 8 in order to avoid such awkwardness, a power loss will be caused at the resistor, and the value of the resistor should be carefully selected with high accuracy so that a sufficiently great coil current may be obtained despite the variation of battery voltage. Further, the current energy stored in the coil 8 after the level of the driving voltage goes low would be discharged through the Zener diode 14, resulting in a greater power loss.
FIG. 4 shows a prior art injector driving circuit designed so that the current flowing through the solenoid coil 8 at operation may be kept constant. This injector driving circuit includes an npn transistor 20 whose collector is coupled to the coil 8 and whose emitter is grounded through a resistor 22, a comparing circuit 24 whose inverted input terminal is coupled to the emitter of the transistor 20, whose non-inverted input terminal is coupled to a reference voltage terminal V.sub.R, and whose output terminal is coupled to the base of the transistor 20, and an npn transistor 26 whose collector is coupled to the base of the transistor 20 through a resistor 28 and whose emitter is grounded. The base of the transistor 26 is coupled with the input terminal V.sub.IN through an inverter 32, and Zener diode 30 is coupled in a reverse direction between the coil 8 and the ground.
In the injector driving circuit shown in FIG. 4, the transistors 26 and 20 are respectively set in conductive and nonconductive states while a low-level driving voltage is being applied to the input terminal V.sub.IN. When a high-level driving signal is applied to the input terminal V.sub.IN, as shown in FIG. 5A, the transistor 26 is turned off, and the output voltage of the comparing circuit 24 is applied to the base of the transistor 20. As a result, a coil current flows through the coil 8, transistor 20, and resistor 22, as shown in FIG. 5B. When a voltage drop at the resistor 22 exceeds a reference voltage V.sub.R, a low-level output voltage is delivered from the comparing circuit 24 to turn off the transistor 20. If the resistance value of the resistor 22 is R1, the coil current is eventually kept at V.sub.R /R1. In this case, the collector voltage of the transistor 20 is reduced to zero level at the leading edge of the driving voltage, as shown in FIG. 5C, and is maintained at a constant value after gradually increasing until the coil current becomes constant. Thereafter, when the driving voltage is reduced to low level, the transistor 20 is turned off, and the coil current flowing at that time flows through the Zener diode 30, and the collector current of the transistor 20 is gradually reduced to a voltage V.sub.D after once increasing to a Zener voltage.
Capable of easily setting the coil current to a relatively small value, the injector driving circuit of FIG. 4 has an advantage over the injector driving circuit shown in FIG. 2. Since the transistor 20 is, however, operated in a linear region, a great power loss will be caused at the transistor 20. Further, the slow rising response of the coil current is not improved yet.
FIG. 6 is a circuit diagram of a prior art injector driving circuit intended to eliminate the drawbacks of the injector driving circuit of FIG. 4. The injector driving circuit of FIG. 6 has the same construction as that of the one shown in FIG. 4 except that, in the circuit of FIG. 6, the reference voltage applied to the comparing circuit 24 is made variable according to the magnitude of the current flowing through the coil 8. Namely, the injector driving circuit of FIG. 6 additionally includes an R-S flip-flop circuit 34 whose set input terminal is coupled to the emitter of the transistor 20 and whose reset input terminal is coupled to the input terminal V.sub.IN through an inverter 32, an npn transistor 36 whose base is coupled to the Q output terminal of the flip-flop circuit 34 and whose emitter is grounded, and three resistors 38, 40 and 42 coupled in series between a power supply terminal V.sub.C and the ground. A junction between the resistors 38 and 40 is coupled to the non-inverted input terminal of the comparing circuit 24, while a junction between the resistors 40 and 42 is coupled to the collector of the transistor 36.
When the driving voltage is at a low level, the transistor 20 is kept in the nonconductive state, as aforesaid, and the flip-flop circuit 34 is brought to a reset state to keep the transistor 36 in the nonconductive state. As a result, a first reference voltage V.sub.R1 is applied to the non-inverted input terminal of the comparing circuit 24. When the driving voltage reaches high level, as shown in FIG. 7A, the transistor 20 is turned on by the high-level output voltage from the comparing circuit 24, as mentioned before, and a current flows through the coil 8, transistor 20, and resistor 22, as shown in FIG. 7B. When the voltage drop at the resistor 22 reaches the first reference voltage V.sub.R1, a low-level output voltage is delivered from the comparing circuit 24 to turn off the transistor 20. At the same time, the flip-flop circuit 34 is set to deliver a high-level output signal as shown in FIG. 7C from its Q output terminal, thereby causing the transistor 36 to conduct. Thus, a second reference voltage V.sub.R2 lower than the first reference voltage V.sub.R1 is applied to the non-inverted input terminal of the comparing circuit 24, as shown in FIG. 7D. Accordingly, the coil current is kept constant at V.sub.R2 /R1. As shown in FIG. 7E, the collector voltage of the transistor 20 changes in the same manner as in the case of the injector driving circuit of FIG. 4.
In the injector driving circuit of FIG. 6, the high-level reference voltage V.sub.R1 is used in the initial stage, so that the rise of the coil current can be made steep by suitably determining the electrical characteristics of the coil 8, transistor 20, and resistor 22. Since the reference voltage is changed to the low-level reference voltage V.sub.R2 after the initial stage, the coil current required to keep the coil 8 in the pull-on state can be set to a relatively small value (V.sub.R2 /R1) which depends on the reference voltage V.sub.R2 and the value of the resistor 22. Even in the injector driving circuit shown in FIG. 6, however, the transistor 20 operates in a linear region, so that the problem of power loss at the transistor 20 is not solved yet. Moreover, when the level of the driving voltage again goes low to turn off the transistor 20, the current having so far been flowing through the coil 8 will be discharged through the Zener diode 30 to the ground to be wasted.